Module 1 - Combinational Logic

Jim Duckworth, WPI Module 11

Verilog – Module 1

Introduction and Combinational Logic

Jim Duckworth

ECE Department, WPI

Jim Duckworth, WPI Verilog Module Rev B2

Verilog background

• 1983: Gateway Design Automation released Verilog HDL

“Verilog” and simulator

• 1985: Verilog enhanced version – “Verilog-XL”

• 1987: Verilog-XL becoming more popular (same year

VHDL released as IEEE standard)

• 1989: Cadence bought Gateway

• 1995: Verilog adopted by IEEE as standard 1364

– Verilog HDL, Verilog 1995

• 2001: First major revision (cleanup and enhancements)

– Standard 1364-2001 (or Verilog 2001)

• System Verilog under development

– Better system simulation and verification support


Books

• “FPGA Prototyping by Verilog Examples”, 2008, Pong P. Chu, Wiley 978-0-470-18532-2

• “Verilog by Example – A concise introduction for FPGA Design” by Blaine C. Readler, 2011, Full Arc Press 978-0-9834973-0-1

• “Starters Guide to Verilog 2001” by Ciletti, 2004, Prentice Hall 0-13-141556-5

• “Fundamentals of Digital Logic with Verilog Design” by Brown and Vranesic, 2003, McGraw-Hill, 0-07-282878-7

• “Advanced Digital Design with the Verilog HDL”, by Ciletti, 2003, Prentice-Hall, 0-13-089161-4

• “HDL Chip Design” by Smith, 1996, Doone Publications, 0-9651934-8

• “Verilog Styles for Synthesis of Digital Systems” by Smith and Franzon, 2000, Prentice Hall, 0-201-61860-5

• “Verilog for Digital Design” by Vhadi and Lysecky, 2007, Wiley, 978-0-470-05262-4


Create Verilog Module


Module Created

• No separate entity and arch –

just module

• Ports can be input, output, or

inout

• Note: Verilog 2001 has

alternative port style:– (input a, b, sel, output y);

– Also place in column:

– (

– input a,

– input b,

– input sel,

– output y

– );


Add assign statement

• Similar to VHDL conditional signal assignment – continuous assignment

• Same hardware produced as with VHDL


Verilog - general comments

• VHDL is like ADA and Pascal in style

• Strongly typed – more robust than Verilog

• In Verilog it is easier to make mistakes

• Watch for signals of different widths

• No default required for case statement, etc

• Verilog is more like the ‘c’ language

• Verilog IS case sensitive

• White space is OK

• Statements terminated with semicolon (;)

• Verilog statements between

• module and endmodule

• Comments // single line and /* and */


Verilog and VHDL – Reminder

• VHDL - like Pascal and Ada programming languages

• Verilog - more like ‘C’ programming language

• But remember they are Hardware Description Languages -

They are NOT programming languages

– FPGAs do NOT contain an hidden microprocessor or interpreter or

memory that executes the VHDL or Verilog code

– Synthesis tools prepare a hardware design that is inferred from the

behavior described by the HDL

– A bit stream is transferred to the programmable device to configure

the device

– No shortcuts! Need to understand combinational/sequential logic

• Uses subset of language for synthesis

• Check - could you design circuit from description?

Jim Duckworth, WPI Verilog Module Rev A9

Verilog – Combinational Logic

Verilog for Synthesis


Verilog – logic and numbers

• Four-value logic system

• 0 – logic zero, or false condition

• 1 – logic 1, or true condition

• x, X – unknown logic value

• z, Z - high-impedance state

• Number formats

• b, B binary

• d, D decimal (default)

• h, H hexadecimal

• o, O octal

• 16’H789A – 16-bit number in hex format

• 1’b0 – 1-bit


Verilog types

• Constants– parameter DIME = 10;

– parameter width = 32, nickel = 5;

– parameter quarter = 8’b0010_0101;

• Nets– wire clock, reset_n;

– wire[7:0] a_bus;

• Registers– reg clock, reset_n;

– reg[7:0] a_bus;

• Integer

– only for use as general purpose variables in loops

– integer n;


Operators

• Bitwise– ~ negation Verilog VHDL

– & and y = a & b; y = a AND b;

– | inclusive or y = a | b; y = A OR b;

– ^ exclusive or y = a ^ b; y = a XOR b;

– y = ~(a & b); y = A NAND b;

– y = ~ a; y = NOT a;

• Reduction (no direct equivalent in VHDL)

– Accept single bus and return single bit result

• & and y = & a_bus;

• ~& nand

• | or y = | a_bus;

• ^ exclusive or


Operators (cont’d)

• Relational (return 1 for true, 0 for false)– < less than, <=

– > greater than >=

• Equality– == logical equality

– != logical inequality

• Logical Comparison Operators– ! logical negation

– && logical and

– || logical or

• Arithmetic Operators– +

– -

– *


Operators (cont’d)

• Shift– << logical shift left, (<<< arithmetic)

– >> logical shift right (>>> arithmetic)

• Conditional

– Only in Verilog - selects one of pair expressions

– ? :

– Logical expression before ? is evaluated

– If true, the expression before : is assigned to output

– If false, expression after : is assigned to output

• Y = (A > B) ? 1 : 0

• Y = (A == B) ? A + B : A – B


Simple Combinational Example

View Technology Schematic



Decoder Tutorial Demo Example

sw0

sw1

led0

led1

led2

led3

led4

led5

led6

led7

sw2


Verilog Source Code


Concurrent statements

• VHDL

– Process

– Signal assignments

• Verilog

– always statement

– Continuous assignment - assign


Verilog wire and register data objects

• Wire – net, connects two signals together

– wire clk, en;

– wire [15:0] a_bus;

• Reg – register, holds its value from one procedural

assignment statement to the next

– Does not imply a physical register – depends on use

– reg [7:0] b_bus;


Index and Slice

• VHDL

– Use to and downto to specify slice

– Concatenation &• c_bus(3 downto 0) <= b_bus(7 downto 4);

• c_bus(5 downto 0) <= b_bus(7) & a_bus(6 downto 3) & ‘0’;

• Verilog

– Use colon :

– Concatenation {,}• assign c_bus[3:0] = b_bus[7:4];

• assign c_bus[5:0] = {b_bus[7], a_bus[6:3], 1’b0};


Internal wires

• Declare internal wires:


Sequential Statements

• VHDL

– reside in process statement

• Verilog

– reside in an always statement

– if statements (no endif)

– case statements (endcase)

– for, repeat while loop statements

– Note: use begin and end to block sequential statements


Decoder – always statement

• 2 to 4 decoder with enable

• Combinational logic using always statement with sensitivity list

– similar to VHDL process – for cyclic behavior

– (@) event control operator

– begin .. end block statement

– note reg for y


Decoder (cont’d)

• Combinational logic using always statement with

sensitivity list

– similar to VHDL process – for cyclic behavior

– (@) event control operator

– begin .. end block statement

• Statements execute sequentially

– if statement

– case statement

• Note: case expression can concatenate signals ({,})

– Sensitivity list

• (a or b or c)

• Verilog 2001 allows comma-separated list (a, b, c)

Decoder – CASE statement

• CASE is better for this type of design - no priority

– Exactly same logic produced


Decoder – 3 to 8 with CASE



MUX example

• Example multiplexer with conditional operator

• Selects different values for the target signal

– priority associated with series of conditions

– (similar to an IF statement)

i0

q

i1

i2

i3

a

b


Synthesis Results – Technology Schematic

O = ((I0 * I1 * I3) + (!I0 * I1 * I4) + (!I0 * !I1 * I5) + (I0 * !I1 * I2));

Mux – with CASE statement

• Include all inputs on sensitivity listElaborating module <mux_case>.

WARNING:HDLCompiler:91 - "C:\ece3829\mux_case\mux_case.v" Line 34: Signal <i>

missing in the sensitivity list is added for synthesis purposes. HDL and post-

synthesis simulations may differ as a result.


Mux – fixed sensitivity list

• Exact same logic produced as using conditional operator



Priority Encoder

• Priority Encoder using conditional operator

• Priority order determined by sequence

– similar to if-else statement

Encoder – Technology Schematic

=========================================================================

* HDL Synthesis *

=========================================================================

Synthesizing Unit <encoder>.

Related source file is "C:\ece3829\encoder\encoder.v".

WARNING:Xst:647 - Input <i0> is never used. This port will be preserved and left unconnected if it

belongs to a top-level block or it belongs to a sub-block and the hierarchy of this sub-block is

preserved.

Summary:

inferred 2 Multiplexer(s).

Unit <encoder> synthesized.

===============================================================

HDL Synthesis Report

Macro Statistics

# Multiplexers : 2

2-bit 2-to-1 multiplexer : 2

===============================================================


Add ‘gs’ output


Synthesize - Design Summary

=========================================================================

* Design Summary *

=========================================================================

Clock Information:

------------------

No clock signals found in this design

Asynchronous Control Signals Information:

----------------------------------------

No asynchronous control signals found in this design

Timing Summary:

---------------

Speed Grade: -3

Minimum period: No path found

Minimum input arrival time before clock: No path found

Maximum output required time after clock: No path found

Maximum combinational path delay: 5.456ns

=========================================================================


Implement Design

Device Utilization Summary:

Slice Logic Utilization:

Number of Slice Registers: 0 out of 18,224 0%

Number of Slice LUTs: 2 out of 9,112 1%

Number used as logic: 2 out of 9,112 1%

Number using O6 output only: 1

Number using O5 output only: 0

Number using O5 and O6: 1

Number used as ROM: 0

Number used as Memory: 0 out of 2,176 0%

Slice Logic Distribution:

Number of occupied Slices: 2 out of 2,278 1%

Number of MUXCYs used: 0 out of 4,556 0%

Number of LUT Flip Flop pairs used: 2

Number with an unused Flip Flop: 2 out of 2 100%

Number with an unused LUT: 0 out of 2 0%

Number of fully used LUT-FF pairs: 0 out of 2 0%

Number of slice register sites lost

to control set restrictions: 0 out of 18,224 0%


Creating adder – using LUTs


Technology Schematic


Example of simple mistake

• No errors or warnings!



Top-Down Design Hierarchy

• Instantiate module (counter example with decoder)

module decoder(

input [3:0] count,

output [6:0] seven_seg

);

// instantiate decoder module in counter

// using position of ports

decoder d1 (count_val, seven_seg_val);

// or using formal and actual names

decoder d1 (.count(count_val), .seven_seg(seven_seg_val));


Tri-state example

• Using conditional operator in continuous assignment

Module 1 - Combinational Logic

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